Wednesday, February 1, 2017

The compromise of the Croat Enigma K cipher machine by the German Army’s codebreakers

Signals
intelligence and codebreaking played an important role in WWII. British and
American codebreakers solved many important Axis crypto systems and similarly
their Axis counterparts also had their own successes.

Both the
Allies and the Axis powers solved not only their opponents’ communications but
also those of neutral powers and in some cases the communications of their own
allies.

For example
the German codebreaking agencies solved Japanese and Italian diplomatic ciphers
during the 1930’s and in WWII.

The Germans
also solved the messages of their minor allies. One such case concerns the
Enigma K (commercial version) used by the Armed Forces of the Independent
State of Croatia in WWII.

The
Independent State of Croatia

The Kingdom of Yugoslavia was
one of the states that were created when the old Austro-Hungarian empire
collapsed at the end of WWI. The country covered a large area in the Balkans
but was politically unstable since it was made up of a diverse group of peoples
(Serbs, Croats, Slovenes, Montenegrins).

Although the
Croat State was a puppet state of Germany its communications were targeted by
the German Army’s codebreakers.

The German
Army High Command’s codebreaking department

During WWII
the German Army made extensive use of signals intelligence and codebreaking in
its operations against enemy forces. German commanders relied on signals
intelligence in order to ascertain the enemy’s order of battle and track the
movements of units.

The German
Army’s signal intelligence agency operated a number of fixed intercept stations
and also had mobile units assigned to Army Groups. These units were called KONA
(Kommandeur der Nachrichtenaufklärung) - Signals Intelligence Regiment and each
had an evaluation centre, a stationary intercept company, two long range signal
intelligence companies and two close range signal intelligence companies.

The KONA
units did not have the ability to solve complicated Allied cryptosystems.
Instead they focused on exploiting low/mid level ciphers and even in this
capacity they were assisted by material sent to them by the central
cryptanalytic department. This was the German Army High Command’s Inspectorate 7/VI.

Inspectorate
7/VI had separate departments for the main Allied countries, for cipher
security, cipher research and for mechanical cryptanalysis (using punch card
machines and more specialized equipment).

Yugoslav
communications were also worked on by a small detachment under Lieutenant
Wollny (Nachrichten Aufklärung Zug ‘W’) based in Belgrade and by the units of KONA
4 (Kommandeur der Nachrichtenaufklärung 4 - Signals Intelligence Regiment 4).

The Croat Enigma
K cipher machine

According to
German reports the Croatian forces used several cryptosystems, from Caesar
cipher and simple transposition to 4 and 5-figure enciphered codes.

A more
interesting system identified in Croatian radio traffic was the Enigma K cipher
machine.

The device
worked according to the Enigma principle with a scrambler unit containing an
entry plate, 3 cipher wheels and a reflector. Each of the cipher wheels had a
tyre, marked either with the letters of the alphabet or with the numbers 1-26,
settable in any position relative to the core wheel, which contained the wiring.
The tyre had a turnover notch on its left side which affected the stepping
motion of the device.

The position
of the tyre relative to the core was controlled by a clip called Ringstellung
(ring setting) and it was part of the cipher key, together with the
position of the 3 cipher wheels.

The
commercial version was different from the version used by the German Armed
Forces in that it lacked a plugboard
(stecker). Thus in German reports it was called unsteckered Enigma.

The Croatian
authorities first received 12 Enigma machines in July 1942 (3). More machines
were purchased in 1943 and 1944. In the period November ’43 - March ‘44 Enigmas
that had been used by the Condor Legion in Spain were rewired for the General
Staff of the Croatian Home Defence Forces (Kroatischen
Landwehr – Hrvatsko Domobranstvo) (4).

The reflector
remained on the commercial wiring Ch 11 Tz 86, however the 3 cipher wheels
received the new wiring Ch 11 Tz 364 a–c (5).

In total 12
new Enigma cipher machines and 29 rewired ones were prepared for Croatian use.
In addition it is likely that another 44 machines were also delivered (6).

Solution
of Croat Enigma K communications

Details on
the solution of Croatian messages enciphered on the Enigma K are available from
the postwar interrogation of Army cryptanalyst dr Buggisch and from the
relevant entries in the War Diary of Inspectorate 7/VI.

2.
Solution of Croat Enigma. This was not an outstanding cryptanalytic
achievement. The machine used was the K model, with three wheels and no
stecker. The machines were made for the Croats by the firm of KROSKY and
KRUGER, Berlin, which gave the wirings promptly to OKW/WNV in about 1941 or
1942. A single key was used throughout the entire Croat Army and area, and this
consisted only of a list of 100 settings for a period of a month. As far as
Buggisch knew the Ringstellung stayed always at AAA, and the wheel order at 1,
2, 3. Just to make sure, the Germans paid for one of the first keys used, and
with this decoded traffic were able to establish stereotypes and solve almost
100% from the first.

The
solutions were done entirely by hand with wiring charts, assuming a pet
beginning (one third of all messages began with "MINORS") and
assuming the left hand wheels and Umkehrwalz unmoving (only one notch per wheel
as in the commercial model). The Croats also had pet indicators and so would
furnish depths in case this method did not work. The setting was indicated
directly by a two digit number unenciphered, so that the settings wore solved
almost as fast as they came, and the traffic read currently from then on.
Buggisch did not recall the contents in detail. 90% of it was uninteresting;
there were some interesting messages about actions against Tito

Buggisch
said the Germans had considered equipping the Croats With the military Enigma,
as they did for HUNGARY, ROUMANIA, FINLAND and ITALY (and JAPAN, he
thinks) in about 1942. However, they decided against this as they believed the
corrupt CROATS would go right on selling the keys to British agents, while
they, the GERMANS, would have to pay as well instead of solving free. (The
possibility of a BRITISH solution obviously did not occur to Buggisch during
this discussion of the K model.)

According to
Buggisch one third of all messages began with the word ‘MINORS’. MINORS stood
for ‘Ministarstvo Oruzanih Snaga’ -
Ministry of Armed Forces of the Independent State of Croatia (7).

Thus this was
high level traffic between the Ministry of Armed Forces and the regional
military commands.

The war diary
of Inspectorate 7/VI (8) confirms Buggisch’s statements and shows that in the
period 1943-45 the Croatian Enigma was regularly read by the Germans.

Information
from the War Diary of Inspectorate 7/VI

In June ’43
the report of Referat 13 (security of German cipher machines) said that the use
of the commercial Enigma K machine had been identified in Croatian 5-letter
cipher traffic and by using the wheel wirings supplied by the company Heimsoeth
& Rinke together with known key documents it was possible to break into
this traffic. Presumably ‘known key
documents’ would have been the compromised documents that Buggisch
mentioned in report I-92.

In July ’43
the report of Referat 6 (Balkan countries) said that there was cooperation with
Referat 13 (Wm. Buggisch) on the solution of the Croat Enigma. 23 indicators (for
the initial position of the rotors) were recovered and transmitted to Lieutenant
Wollny’s Nachrichten Aufklärung Zug ‘W’, together with a cipher machine for
processing the accumulating material.

In September
’43, indicator groups continued to be solved and sent to the Wollny unit for
direct exploitation. However according to the Referat 13 report the Croatian
authorities changed the indicator system by having 10 new indicators valid for
each day instead of the previous arrangement for 100 indicators valid for each
quarter.

In October
’43, 13 indicators were sent to the Wollny unit. The report of Referat 13 says
that the current solution of the indicators could be stopped. This would seem
to imply that they got copies of the indicator tables, thus they didn’t need to
solve them cryptanalytically.

In December ’43
there were organizational changes in Inspectorate 7/VI, with Referat 6 becoming
Referat a5 and Referat 13 becoming Referat b2.

New
indicators were recovered by departments a5 and b2. The report of b2 says that
the solution of the Croat Enigma indicators had to be resumed because procurement
was not yet possible in the new key period.

Interception
resumed in July ’44 with the report pointing out the negative consequences of
losing touch with this traffic (‘The
three-month interruption of interception makes itself felt in a disadvantageous
manner’)

The files of
Inspectorate 7/VI, listed in TICOM report IF-272 - TAB ‘D’, include the
following report on the Croatian Enigma in page 8:

Zusammengefasste Umkehrwalze.
Kroatien. AA-AZ.

Walze I. II. III.

These should
be the wirings for the reflector and the three wheels.

Acknowledgements: I have to thank Frode Weierud for his help in
translating the relevant passages from the War Diary of Inspectorate 7/VI and
for the information on the Croat Enigma orders and dr Nikica Baric for
explaining the meaning of the term MINORS.